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Fig. 1. Schematic diagram of the lateral and in-depth spreads of a smouldering wildfire in a layer of peat. Various kinetic schemes of different complexity have been pro- posed for smouldering combustion. Ohlemiller [1] proposed a 3-step and 3-species scheme, including one pyrolysis and two oxidations, as general scheme for any smoulder-prone fuel. Kashiwagi and Nambu | 13] quantified the kinetic parameters of this scheme for cel- lulose using thermogravimetric analysis (TGA) on small samples (~mg scale) under nitrogen and air atmospheres. Rein et al. [9] stud- ied polyurethane foam, and extended Ohlemiller’s scheme to 5-step and 4-species kinetics (two pyrolysis and three oxidations). This ex- tended scheme allows explaining the reaction structure of a smoul- dering front in both forward and opposed propagation. In doing so, Rein et al. [9] developed a methodology where a genetic algorithm

Figure 1 Schematic diagram of the lateral and in-depth spreads of a smouldering wildfire in a layer of peat. Various kinetic schemes of different complexity have been pro- posed for smouldering combustion. Ohlemiller [1] proposed a 3-step and 3-species scheme, including one pyrolysis and two oxidations, as general scheme for any smoulder-prone fuel. Kashiwagi and Nambu | 13] quantified the kinetic parameters of this scheme for cel- lulose using thermogravimetric analysis (TGA) on small samples (~mg scale) under nitrogen and air atmospheres. Rein et al. [9] stud- ied polyurethane foam, and extended Ohlemiller’s scheme to 5-step and 4-species kinetics (two pyrolysis and three oxidations). This ex- tended scheme allows explaining the reaction structure of a smoul- dering front in both forward and opposed propagation. In doing so, Rein et al. [9] developed a methodology where a genetic algorithm

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pyrolysis and oxidation reactions [1,3,9,12]. The endothermic pyrolysis reactions decompose the virgin fuel into pyrolysate gases and char. estimate that the average emission from peat fires is roughly equivalent to 15% of the man-made emissions [6]. Moreover, the atmospheric release of ancient carbon from the soil and the sensi- tivity of peat ignition to higher temperatures and drier conditions create a positive feedback mechanism to climate change [3].
pyrolysis and oxidation reactions [1,3,9,12]. The endothermic pyrolysis reactions decompose the virgin fuel into pyrolysate gases and char. estimate that the average emission from peat fires is roughly equivalent to 15% of the man-made emissions [6]. Moreover, the atmospheric release of ancient carbon from the soil and the sensi- tivity of peat ignition to higher temperatures and drier conditions create a positive feedback mechanism to climate change [3].
Fig. 2. The composition of peat and a possible decomposition paths and products. From the viewpoint of fire behaviour, the most important com- ponents of peat are the organic matter (OM), water and minerals [16,17]. The left hand side of Fig. 2 shows the relative amounts of the different components found in typical peat samples, although the proportions can vary significantly with the ecosystem type (i.e. boreal, temperate or tropical), originating vegetation (e.g., sphagnum or feather) and depth (i.e. age and level of decomposi- tion) [18]. Generally, carbon is one of the most abundant chemical elements and its fraction in peat is between 30% and 65% [18], sim- ilar to common biomass types [19] and most synthetic polymers [20].
Fig. 2. The composition of peat and a possible decomposition paths and products. From the viewpoint of fire behaviour, the most important com- ponents of peat are the organic matter (OM), water and minerals [16,17]. The left hand side of Fig. 2 shows the relative amounts of the different components found in typical peat samples, although the proportions can vary significantly with the ecosystem type (i.e. boreal, temperate or tropical), originating vegetation (e.g., sphagnum or feather) and depth (i.e. age and level of decomposi- tion) [18]. Generally, carbon is one of the most abundant chemical elements and its fraction in peat is between 30% and 65% [18], sim- ilar to common biomass types [19] and most synthetic polymers [20].
Characteristics of four peat samples. * Estimated from this study (dry basis). Table 1
Characteristics of four peat samples. * Estimated from this study (dry basis). Table 1
Fig. 3. (a) mass, and (b) mass-loss rate of CH peat in nitrogen (wet basis) as a function of temperature for three heating rates. Marks: experimental data [15], and lines simulations.
Fig. 3. (a) mass, and (b) mass-loss rate of CH peat in nitrogen (wet basis) as a function of temperature for three heating rates. Marks: experimental data [15], and lines simulations.
Fig. 4. (a) mass, and (b) mass-loss rate of CH peat in air (wet basis) as a function of temperature for three heating rates. Marks: experimental data [15], and lines: simulation: Note that the scale in Fig. 4b is different from that in Figs. 3b.
Fig. 4. (a) mass, and (b) mass-loss rate of CH peat in air (wet basis) as a function of temperature for three heating rates. Marks: experimental data [15], and lines: simulation: Note that the scale in Fig. 4b is different from that in Figs. 3b.
* Calculated from Eq. (11). Kinetic and stoichiometric parameters for the CH peat sample with the 5-stey scheme. be seen in Fig. 4b, above 550 K, the oxidation of peat or char is al- ready dominant, and pyrolysis does not play an important role. Table 2
* Calculated from Eq. (11). Kinetic and stoichiometric parameters for the CH peat sample with the 5-stey scheme. be seen in Fig. 4b, above 550 K, the oxidation of peat or char is al- ready dominant, and pyrolysis does not play an important role. Table 2
Fig. 5. Simulation results of the TG experiment for CH peat at k = 10 K/min, (a) reaction rates, @,, and (b) dry-basis mass fractions, [mj], in nitrogen; (c) reaction rates, @;, and (d) dry-basis mass fractions, [m;], in air. The rate of peat oxidation is scaled down by 1/5 due to its exceptionally high peak. tions, [m;], at k = 10 K/min are shown in Fig. 5. T The simulated reaction rates, «;, and the dry-basis mass frac- he first peak o' mass-loss rate in both nitrogen (Fig. 3b) and air (Figs. 4b) between 300 and 400 K, is correctly simulated by Eq. (1) as t he drying stage In nitrogen (Fig. 3b), the subsequent peak is simulated by Eq. (2) as t t ( i t imultaneous pyrol d S the subsequent f-c r ‘ange (500-570 k), he peat pyrolysis. No clear overlapping between d lysis is observed, w ecomposition during drying. In air (Fig. 5b), hich confirms the assumption o ysis and oxidation reactions of rying and pyro- f negligible peat after drying, the peat as well a: har oxidation overlap in a narrow temperature producing the highest peak of in Fig. 4b. Comparison between nitrogen and air simulations shows hat in TG, oxidations dominate the peat decomposition Cn /@pp* = 19). Figure 5d shows that the production of f-chai Ss larger’ than that of a-char (mp*/my* = 11). Thi hat above 500 K the oxidation rate of f-char is larger than that mass-loss rate is also explains
Fig. 5. Simulation results of the TG experiment for CH peat at k = 10 K/min, (a) reaction rates, @,, and (b) dry-basis mass fractions, [mj], in nitrogen; (c) reaction rates, @;, and (d) dry-basis mass fractions, [m;], in air. The rate of peat oxidation is scaled down by 1/5 due to its exceptionally high peak. tions, [m;], at k = 10 K/min are shown in Fig. 5. T The simulated reaction rates, «;, and the dry-basis mass frac- he first peak o' mass-loss rate in both nitrogen (Fig. 3b) and air (Figs. 4b) between 300 and 400 K, is correctly simulated by Eq. (1) as t he drying stage In nitrogen (Fig. 3b), the subsequent peak is simulated by Eq. (2) as t t ( i t imultaneous pyrol d S the subsequent f-c r ‘ange (500-570 k), he peat pyrolysis. No clear overlapping between d lysis is observed, w ecomposition during drying. In air (Fig. 5b), hich confirms the assumption o ysis and oxidation reactions of rying and pyro- f negligible peat after drying, the peat as well a: har oxidation overlap in a narrow temperature producing the highest peak of in Fig. 4b. Comparison between nitrogen and air simulations shows hat in TG, oxidations dominate the peat decomposition Cn /@pp* = 19). Figure 5d shows that the production of f-chai Ss larger’ than that of a-char (mp*/my* = 11). Thi hat above 500 K the oxidation rate of f-char is larger than that mass-loss rate is also explains
Fig. 6. Interdependence among the kinetic parameters: (a) lg(A,) against E;,, and (b) n, against E,; data from good solutions satisfying A® < 0.1%. The inserted sub- figures have different scales. Finding the right level of complexity is a key question in kinet- ics modelling. In order to explore this issue, a reduced 3-step showing a high linearity as measured by the R’ coefficient, except for the «-char oxidation. The reaction order, nm, also depends on E, for all reactions (see Fig. 6b). Except for nyo, all other n, increases linearly with E;, although the scatter of data is significant. There- fore, the kinetic triplet (A,, E,, and n,) are interdependent, as math- ematically proved for TG conditions by [37]. For «-char oxidation, the most likely reason for the non-linear dependence of the triplet is that its very low reaction rate carries a low contribution to the to- tal mass loss measured in TG. This is a limitation in the experimen- tal data available that cannot provide sufficiently information to fix the temperature range and reaction rate of «-char oxidation accu- rately. Therefore, this serves as evidence that adding more reaction steps would not improve the interpretation of this TG data.
Fig. 6. Interdependence among the kinetic parameters: (a) lg(A,) against E;,, and (b) n, against E,; data from good solutions satisfying A® < 0.1%. The inserted sub- figures have different scales. Finding the right level of complexity is a key question in kinet- ics modelling. In order to explore this issue, a reduced 3-step showing a high linearity as measured by the R’ coefficient, except for the «-char oxidation. The reaction order, nm, also depends on E, for all reactions (see Fig. 6b). Except for nyo, all other n, increases linearly with E;, although the scatter of data is significant. There- fore, the kinetic triplet (A,, E,, and n,) are interdependent, as math- ematically proved for TG conditions by [37]. For «-char oxidation, the most likely reason for the non-linear dependence of the triplet is that its very low reaction rate carries a low contribution to the to- tal mass loss measured in TG. This is a limitation in the experimen- tal data available that cannot provide sufficiently information to fix the temperature range and reaction rate of «-char oxidation accu- rately. Therefore, this serves as evidence that adding more reaction steps would not improve the interpretation of this TG data.
* Calculated from Eq. (11). Kinetic and stoichiometric parameters for Scottish and Siberian peat samples with the 4-step scheme of peat decomposition. Table 3
* Calculated from Eq. (11). Kinetic and stoichiometric parameters for Scottish and Siberian peat samples with the 4-step scheme of peat decomposition. Table 3
Fig. 7. Mass-loss rate of CH peat in air (k =10K/min) simulated by chemical schemes with different number of steps. The number of steps in the legend includes the drying plus the peat decomposition steps.
Fig. 7. Mass-loss rate of CH peat in air (k =10K/min) simulated by chemical schemes with different number of steps. The number of steps in the legend includes the drying plus the peat decomposition steps.
Fig. 8. The wet-basis mass loss (up) and mass-loss rate (down) of (a) SC, (b) SI-A, and (c) SI-B samples in air for three heating rates. Marks: experimental data [14], and lines simulations.
Fig. 8. The wet-basis mass loss (up) and mass-loss rate (down) of (a) SC, (b) SI-A, and (c) SI-B samples in air for three heating rates. Marks: experimental data [14], and lines simulations.
Fig. 9. Modelled reaction rates, @, (top) and species mass fraction, m; (bottom) for (a) SC, (b) SI-A, and (c) SI-B samples in air (k = 20 K/min) The influence of the number of reactions is also discussed here. Figure 10 compares the original 4-step decomposition scheme in Eqs. (2)-(5) with the 3-step scheme in Eq. (15) and the 2-step scheme in Eq. (16) by modelling the mass-loss rate at 20 K/min. Both the 3-step and 2-step schemes can roughly capture the two peaks of mass-loss rate, but the agreement is poor, especially for high-moor SC and SI-A peat. The difference in the degree of fit be- tween 2-step and 3-step schemes is very tiny (difficult to discrim- inate for SC and SI-A samples in Fig. 10). Moreover, the simulation also reveals that with 3-step or 2-step kinetics the disagreement further increases in the blind prediction of other heating rates. The simulated reaction rates, @,, and the dry-basis mass frac- tions, m;, at 20 K/min are explored in Fig. 9. Similar to the results of CH samples, the rates of peat pyrolysis and oxidation peak at 550-600 K, and then rates of two char oxidations peak at about 700 K. But unlike the CH sample which has a large peat-oxidation rate and a small char-oxidation rate, these high-OC samples show that the maximum value of all reactions rates are in the same order of magnitude. Also, «-char and f£-char oxidations play a similarly important role at high temperature. These suggest that the differ- ences in reactivity seen between CH and SC/SI samples are due to the different carbon contents and decomposition degrees.
Fig. 9. Modelled reaction rates, @, (top) and species mass fraction, m; (bottom) for (a) SC, (b) SI-A, and (c) SI-B samples in air (k = 20 K/min) The influence of the number of reactions is also discussed here. Figure 10 compares the original 4-step decomposition scheme in Eqs. (2)-(5) with the 3-step scheme in Eq. (15) and the 2-step scheme in Eq. (16) by modelling the mass-loss rate at 20 K/min. Both the 3-step and 2-step schemes can roughly capture the two peaks of mass-loss rate, but the agreement is poor, especially for high-moor SC and SI-A peat. The difference in the degree of fit be- tween 2-step and 3-step schemes is very tiny (difficult to discrim- inate for SC and SI-A samples in Fig. 10). Moreover, the simulation also reveals that with 3-step or 2-step kinetics the disagreement further increases in the blind prediction of other heating rates. The simulated reaction rates, @,, and the dry-basis mass frac- tions, m;, at 20 K/min are explored in Fig. 9. Similar to the results of CH samples, the rates of peat pyrolysis and oxidation peak at 550-600 K, and then rates of two char oxidations peak at about 700 K. But unlike the CH sample which has a large peat-oxidation rate and a small char-oxidation rate, these high-OC samples show that the maximum value of all reactions rates are in the same order of magnitude. Also, «-char and f£-char oxidations play a similarly important role at high temperature. These suggest that the differ- ences in reactivity seen between CH and SC/SI samples are due to the different carbon contents and decomposition degrees.
Fig. 10. Mass-loss rate of (a) SC, (b) SI-A, and (c) SI-B peat in air (20 K/min) simulated by kinetics with different steps.
Fig. 10. Mass-loss rate of (a) SC, (b) SI-A, and (c) SI-B peat in air (20 K/min) simulated by kinetics with different steps.
Fig. 11. Spread modes of 1-D smouldering combustion: (a) lateral spread; and (b) in-depth spread. See Fig. 1 for a combined illustration of these fronts.
Fig. 11. Spread modes of 1-D smouldering combustion: (a) lateral spread; and (b) in-depth spread. See Fig. 1 for a combined illustration of these fronts.
Parameters used in plug flow model.
Parameters used in plug flow model.
Fig. 12. Reaction-zone structure of the lateral spread for (a) the CH peat; and (b) the SC peat.
Fig. 12. Reaction-zone structure of the lateral spread for (a) the CH peat; and (b) the SC peat.
Fig. 13. Reaction-zone structure of the in-depth spread for (a) the CH peat, and (b) the SC Peat.
Fig. 13. Reaction-zone structure of the in-depth spread for (a) the CH peat, and (b) the SC Peat.
Related topics:
Fire BehaviourChemical KineticsPeat Soil
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